Which wheel does the Speedometer key off of?

Hmmm, I had assumed it was linked to the transmission, like most cars. 'Like most cars' is not a very smart thing to assume with the Prius, though...

 

The brake ECU [which also handles ABS, etc] sends a Vehicle Speed
output to the speedometer electronics. Exactly how it derives
that signal from the four wheel-speed sensors is unclear, but I
would hope that it's an average of at least two of them.
.
_H*

 

I am betting the poster used to drive VWs, which used a cable keyed to the left front wheel hub. Most other cars have long gone to transmission gear take-offs.

Not planning a Ferris Buehler experiment, are you?

 

Worked great on my '68 Mustang! Parents were gone on vacation, I was grounded from the car (every last bit of which I paid for myself, inc. insurance, gas etc.). I knew my Dad was the mileage checking type, so, appropriate measures were taken. Bailing wire may have been involved.

 

Resurrecting this thread ... because I've got an answer now: Speed
[and distance] is determined by averaging the sensors from the
two front wheels. http://techno-fandom.org/~hobbit/cars/blocks/
.
_H*

 

After changing the front wheel bearings, and having to take off the dust covers that surround the axles going into the steering knuckle from behind, I am learning that these dust covers have to be positioned appropriately to transmit the signal. The sensors. I lost speedometer because I wasn't aware of that. One poster said that both front wheels are involved, the average speed being the display speed. Speed. So now I have to take both Wheels apart again to correctly place the dust covers